Monitoring method and system for a tool-holding spindle
Patent 7393311 Issued on July 1, 2008. Estimated Expiration Date: 
November 23, 2027. Estimated Expiration Date is calculated based on simple USPTO term provisions. It does not account for terminal disclaimers, term adjustments, failure to pay maintenance fees, or other factors which might affect the term of a patent.
November 23, 2027. Estimated Expiration Date is calculated based on simple USPTO term provisions. It does not account for terminal disclaimers, term adjustments, failure to pay maintenance fees, or other factors which might affect the term of a patent.Patent References 3520228 High speed motorized spindle with tool clamping/unclamping mechanism Tool clamping device for main spindle Machine tool with bar-spindle and DIN standard toolholder changer Magnetic bearing spindle device for machine tools Electric drive spindle Device of a tool spindle Apparatus for detecting position of drawbar in automatic tool change device Spindle for a machine tool with improved tool ejection feature Patent #: 7318692 InventorsAssigneeApplicationNo. 11984851 filed on 11/23/2007US Classes:483/1, PROCESS483/12, WITH SIGNAL OR INDICATOR409/131, Process409/233And draw barExaminersPrimary: Cadugan, EricaAttorney, Agent or FirmForeign Patent References
International ClassesB23Q 3/157B23C 5/26 DescriptionThis invention is concerned with a method and a system for monitoring a tool-holding spindle in a tool machine, in order to determine whether a tool picked by the spindle from a tool crib in preparation to amachining operation has been properly gripped in its collet.BACKGROUND OF THE INVENTION When a machine cycle is to be repeatedly carried out with different tools, tool machines are programmed for operating a spindle through a working cycle such that the spindle will pick a chosen tool from a tool crib, will clamp its collet on thetool shank, will carry out the programmed operation on the workpiece and finally will return the tool to the tool crib. Two facing, annular registering surfaces on the spindle and the tool, respectively, axially abut with each other to insure that thetool is properly coupled with the spindle, so that a good degree of accuracy and repeatability is achieved. However, minute chips may deposit on the registering surface during machining, which often stick to it, due to the capillarity of the cooling/lubricating fluid that is always present in the area. If the chip fragment is very small, of the orderof a few hundredths of a millimeter (a few tens of microns) thickness, the compressed air usually blown over the spindle before picking the tool may be unable to remove it, and the chip will therefore be pinched between the abutted registering surfaces,thereby preventing the collet from closing, or, alternatively, causing a positioning error, generally involving a lack of coplanarity of the registering surfaces, whereby the tool is inclined with respect to the axis of the spindle. It is known to provide the spindle head with a proximity sensor arranged for detecting the axial position of a control rod controlling the opening and closing of the collet, and for notifying a collet-clamping error in case of an incompletestroke of the rod. However, such sensor is only able to notify coarse errors, due to the unavoidable variation in the nominal end of stroke of the control rod in different tools. Consequently, the above sensor is unable to detect very small fragments,which, however, as stated above, do cause a deviation of the tool axis from the spindle axis, with consequent linear errors amounting up to several tenths of a millimeter at the tool tip. SUMMARY OF THE INVENTION The aim of the present invention is to provide a method and a system for monitoring a tool-holding spindle in a tool machine, in order to determine whether a tool has been properly gripped in the spindle collet without any foreign particlescoming in between, however small. The above aim, as well as other objects and advantages such as will appear from the following disclosure, is attained by the invention by a method for monitoring tool-holding spindles, having the features recited in claim 1, the claims dependentfrom it reciting other advantageous though secondary features. The invention also concerns a system for carrying out the above method, as recited in claim 7. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in more detail below, with reference to a preferred embodiment, shown by way of nonlimiting example in the attached drawings, wherein: FIG. 1 is a view in axial cross-section, partly schematic, of a tool-holding spindle belonging to the system of the invention, holding no tool; FIG. 2 is a view similar to FIG. 1, showing the spindle with unclamped tool; FIG. 3 is a view similar to FIG. 1, showing the spindle with clamped tool; FIG. 4 is a flowchart showing the monitoring method according to a preferred embodiment of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to FIGS. 1-3, a spindle 10 is journaled in a tool machine 12 through ball bearings 14. Spindle 10 is hollow and has, at its operating end, a frusto-conical socket 16 and a coaxial flat, annular registering surface 18 which canreceive a frusto-conical shank 20 of a tool (FIGS. 2 e 3). Shank 20 has an axial, shaped recess 22, which, also in a way known per se, can be engaged by a clamping member or collet comprising a plurality of tongues or reeds 24, which can be radiallywithdrawn or expanded by the longitudinal shifting of a mushroom-shaped head 26 carried at the end of a control rod 28, which is axially shiftable within the spindle. A compression spring 30 biases rod 28 to a backward position in which head 26 expandsthe collet outwardly. A hydraulic drive (not shown) is able to push rod 28 forward, as known per se, against the bias of spring 30, thereby withdrawing reeds 24 to a contracted condition which frees tool 20. An analog proximity sensor 36, which may becapacitive or inductive, is mounted in a stationary position in the tool machine, so that it will interact with a conical surface 38 that is coaxial and integral with control rod 28. The sensor can therefore sense the axial position of the rod bymeasuring the distance from the conical surface. In order to load a tool, the spindle, with spread-out collet as shown on FIG. 1, moves forward in front of the shank 20 of a desired tool carried in a tool crib (not shown), and the control rod 28 is then pushed forward by the hydraulical drivein order to withdraw the locking reeds 24 and thereby allow the shank (as shown on FIG. 2) to move into the frusto-conical socket 16, until the annular surface 18 of the spindle abuts against a corresponding annular surface on the shank. Control rod 28is then released and is shifted by spring 30 (FIG. 3) in order to spread the collet reeds and clamp the tool. If a chip fragment of about a tenth of a millimeter thickness is pinched between the two facing annular surfaces, the fragment will cause an axial rod position error that is generally smaller than the error threshold measurable by sensor 36,although the error will affect the machining accuracy, because the tool, due to the chip, will take a slightly inclined position with respect to the spindle axis, and the linear error at the tip of the tool will be amplified. The method of the invention is illustrated in the flowchart of FIG. 4. Whenever a given tool is picked from a tool crib (not shown), the tool is allowed to be clamped, and the current position Pc of the control rod is then measured by thesensor 36. Preferably, a measurement of the current position Pc is obtained by taking a set of successive readings Pc1, Pc2, . . . , Pcn, at close time intervals, e.g. five readings (not shown in detail in FIG. 4 for simplicity),starting with a predetermined delay T after checking that the closure of the collet has been completed, and then by assuming the average of the readings as measurement Pc. The number of readings is chosen as an acceptable compromise between thetime required to perform the measurement and the accuracy of the measurement. The interval between any two successive readings is chosen, e.g., as 10-15 msec, and the delay time T is chosen such that the transients are masked and the sensor output isstabilized, typically as 100 msec. The value of the current position Pc is then compared with the position Pm that was obtained and stored for the same tool in the previous cycle. If the two values Pm and Pc differ by less than a predetermined deviation S, thecollet closure is regarded as regular and the work cycle is allowed to proceed, while the current value Pc is substituted for the stored value Pm. If, on the other hand, the difference between the two values Pm and Pc is larger thanthe deviation threshold S, the closure is considered to be anomalous, the tool machine is stopped, and an alarm signal for operator is emitted. Alternatively, and in fact preferably, as shown in FIG. 4, whenever the above comparison detects a defective closure, rather than immediately stopping the tool machine, the collet is released, the spindle is backed off from the tool crib and thecollet is again clamped, the above described position measurement is repeated, and the stored position Pm is again compared with current position Pc. This attempt is repeated a few times, e.g. four times as shown, and if the comparisoncontinues to have an anomalous issue, the tool machine is stopped at the fourth failure, and the operator is warned by means of an alarm signal. It has in fact been found that a repeated release and clamping of the collet is often sufficient to removeor crumble the fragment, thereby detaching it from the surface to which it was clinging. At the beginning of each working session, when a previous position has not yet been stored for a tool, the value for the previous position Pm is set to zero, and it is therefore impossible to make a comparison. Accordingly, when firstpicking a tool the collet closure is always assumed to be regular, and the current rod position Pc is stored directly as a value for the previous rod position Pm. This step is equivalent to abstain from monitoring the first tool picking, andmay give rise not only to a defective tool alignment in the first machining operation performed, but also to an erroneous measurement in the second operation that will be carried out by the same tool. Such a circumstance is highly unlikely, and will becorrected, in any case, starting with the third operation. In a typical case given by way of example, the proximity sensor has a sensitivity of a 10 mV variation of the generated voltage for each 10 μm variation in the distance from the observed surface. The threshold S is chosen at 25 mV, whichnominally correspond to the ability to sense a thickness of about 25 μm, although in practice only foreign particles of at least 40 μm thickness can be guaranteed to be detected. It is understood that changes can be made in the preferred embodiment of the method as described above, particularly with respect to the values of the threshold S and the delay T, as well as in the number of readings taken by the sensor forcomputing an average, and also in the number of tool-picking attempts made. Moreover, it should be noted that, for the sake of simplicity, no mention has been made of a number of options and functions that are normally available in the numeric controlof tool machines, such as displays, possibility for the operator to activate or suppress desired functions, change parameters such as S or T, and the like. The disclosures in European Patent Application No. 06425871.8 from which this application claims priority are incorporated herein by reference. Field of SearchPROCESSWITH SAFETY MEANS WITH CONTROL MEANS ENERGIZED IN RESPONSE TO ACTIVATOR STIMULATED BY CONDITION SENSOR Responsive to tool Including means to monitor and control, i.e., adaptive machining WITH SIGNAL OR INDICATOR WITH MEANS TO CONDITION OR ADJUST TOOL OR TOOL SUPPORT And draw bar With cutter holder Cutter spindle or spindle support Process Including infeeding With means to protect operative or machine (e.g., guard, safety device, etc.) PROCESSES With tool-retaining means Tool adapter WITH CONTROL MEANS ENERGIZED IN RESPONSE TO ACTIVATOR STIMULATED BY CONDITION SENSOR |
